Human rights activists have raised serious concerns about the health of Hossein Ronaghi Maleki, an Iranian blogger on hunger strike in protest at his 15-year jail sentence and the authorities' refusal to grant him prison leave despite severe medical conditions.

Ronaghi Maleki, 27, was arrested in December 2009 following the unrest in the aftermath of Iran's disputed presidential elections. He was picked up from his father's house in the city of Malekan in Iran's province of East Azerbaijan and taken to Tehran's notorious Evin prison where he spent 376 days in solitary confinement.

Maleki who blogged under the nickname Babak Khorramdin (a reference to an ancient Persian freedom fighter from Azerbaijan) was sentenced to 15 years in jail in 2010 on charges of "spreading propaganda against the regime", "membership of the internet group Iran Proxy" and "insulting the Iranian supreme leader [Ayatollah Ali Khamenei] and the president [Mahmoud Ahmadinejad]".

His sentence has sparked outrage among human rights groups including Amnesty International, which said he is "held solely on account of his peaceful exercise of his right to freedom of expression".

While in jail, Maleki has developed a kidney disease and undergone at least four operations. He is currently kept in Tehran's Hashemi-Nejad hospital awaiting another operation. He started his latest hunger strike on May 26.

According to the blogger's father, Ahmad Ronaghi Maleki, the authorities have so far refused to grant him medical leave despite receiving a doctor's order to do so.

This week more than 100 of Maleki's fellow prisoners in Evin wrote a letter to the authorities warning that the blogger would die if not provided with appropriate medical care. In their letter, the prisoners warned Maleki might experience the same fate of Hoda Saber, a political prisoner who died last June on hunger strike.

Hoda Saber, a 52-year-old activist from the opposition Nationalist-Religious movement, died after a cardiac complication, which his wife claimed at the time that was brought on by his hunger strike.